1. Field
The disclosed concept pertains generally to zone selective interlocking and, more particularly, to zone selective interlocking test methods. The disclosed concept also pertains to zone selective interlocking test apparatus. The disclosed concept further pertains to circuit interrupters and power distribution systems including circuit interrupters.
2. Background Information
Circuit interrupters, such as for example and without limitation, circuit breakers, are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition, a short circuit, or another fault condition, such as an arc fault or a ground fault. Molded case circuit breakers typically include a pair of separable contacts per phase. The separable contacts may be operated either manually by way of a handle disposed on the outside of the case or automatically in response to a detected fault condition. Typically, such circuit breakers include an operating mechanism, which is designed to rapidly open and close the separable contacts, and a trip mechanism, such as a trip unit, which senses a number of fault conditions to trip the breaker automatically. Upon sensing a fault condition, the trip unit trips the operating mechanism to a trip state, which moves the separable contacts to their open position.
Zone selective interlocking (ZSI) (e.g., also known as “zone interlocking”) is a method of controlling circuit breakers in order to provide selectivity with relatively very short delay times, irrespective of the number of zones (e.g., without limitation, a line side zone; a load side zone; a number of upstream zones; a number of downstream zones; a number of grading levels) and the location of a fault in a power distribution system. A ZSI input and a ZSI output are provided at each circuit breaker. Interlocking may be applied to faults between phases or earth-faults or both.
As one example, zone interlocking uses a communication scheme to connect line and load circuit breaker trip units together. When a fault occurs, the trip units communicate to determine which load side circuit breaker is closest to the fault. The trip unit in the circuit breaker closest to the fault overrides any customer-defined delay and opens instantaneously, thereby clearing the fault and allowing the line side circuit breakers to remain closed.
If ZSI is used in several zones, then each circuit breaker affected by, for example, a short circuit current (i.e., upstream of the fault) interrogates the circuit breaker(s) directly downstream of that affected circuit breaker to determine whether the short circuit current is present in or is affecting the adjacent downstream zone. A delay setting tZSI is adjusted at each circuit breaker to ensure that the downstream circuit breaker, directly upstream of the fault, has time to interrupt the fault current. The advantages of ZSI increase with additional zones, since time-based selectivity can result in unacceptably long delays at the upstream power source end of the system.
Several examples of the operation of ZSI are discussed in connection with FIG. 1, which shows an example power distribution system using multiple power sources in the upstream ZONE 1. In this example, there are two downstream zones, ZONE 2 and ZONE 3, although any suitable number of downstream zones can be employed. As a first example, there is a fault, such as a short circuit, at position 3. Circuit breakers CB1, CB2, CB3, CB5 and CB7 detect the short circuit. CB7 blocks CB5 by the ZSI OUT signal of CB7 and, as a result, also CB1, CB2 and CB3, in order that they do not trip for tZSI=50 ms. Since CB7 does not receive a blocking ZSI IN signal from a subordinate, downstream circuit breaker, CB7 is responsible for interrupting the short circuit as quickly as possible. In the event of a problem with circuit breaker CB7 (e.g., because CB7 is no longer operational), then upstream CB5, as a back-up, trips after its short time delay setting, tSD=150 ms.
As a second example, there is a short circuit at position 2. Circuit breakers CB1, CB2, CB3 and CB5 detect the short circuit, but CB7 does not. For this reason, CB5 does not receive a blocking ZSI IN signal from CB7, but provides a blocking ZSI OUT signal to CB1, CB2 and CB3. This information tells CB5 that it is the closest breaker upstream of the short circuit. CB5 trips with a delay of tZSI=50 ms instead of with a delay of tSD=150 ms. Here, the clearance time is reduced by 100 ms (=tSD−tZSI=150 ms−50 ms).
As a third example, there is a short circuit at position 1. Only circuit breakers CB1, CB2 and CB3 detect the short circuit and they do not receive a blocking ZSI IN signal from any circuit breaker at a subordinate, downstream zone. For this reason, CB1, CB2 and CB3 trip after tZSI=50 ms. Here, the time saved is 250 ms (=tSD−tZSI=300 ms−50 ms).
There is no known system to fully test and properly verify a zone selective interlocking system.
There is room for improvement in zone selective interlocking.
There is also room for improvement in circuit interrupters and power distribution systems including circuit interrupters, which employ zone selective interlocking.